How many fans are necessary for a computer?

If it's a basic computer how many fans do you need? What if it's a workstation? Or a gaming computer? Is one fan  in the back better than one in the front? what about their size? which the best placement and size? Is one case fan good if you don't plan on overclocking? will the other motherboard parts slow down if you don't have proper fans? Will dust accumulate more often if the fan size, placement, and orientation are not as optimal as they can be? Can someone help me?

put as many fans as you can fit..... and the larger the fans, the better

well, you want to have more intake than exhaust to keep positive air pressure and dust out of your case.

you most likely won't need a whole lot of fans. like 2 120mm intakes in the front and one exhaust at the back/top will be fine in most situations.

A basic computer( with an IGPU) can have no fans at all( if you dont count the one on the CPU and PSU).Workstation I'd put at least 4. Gaming at least 3. But it all depends on the parts, too. And if the case has good airflow. It's always good to have a lot of fans.

Like Cru3L139 said, if you use any fans at all you'll want more intake than exhaust to keep positive air pressure. Although, you only need as many fans as is required to keep temperatures appropriate. If you can keep all your components under 50 C with only passive cooling, then go for it.